While summer is the period of growth and harvest for apple trees, winter is an important time to protect and prune them. Trees need winter care to develop well and may not survive without it. Winter pruning is essential, because it creates the shape that produces strong limbs, permits air movement through the leaves, and encourages fruit formation.
On even the coldest winter day, sunlight can warm the dark bark of a tree enough to soften the living cambium layer that lies under the bark. When night-time temperatures drop, the cambium tissue will freeze, causing injury or tissue death. Failure to protect against freeze injury, or sunscald, is a common cause of winter-kill losses. The most frequent sign of freeze injury is split bark.
To protect against freeze injury, trunk guards are used during the first two or three years of growth. In later years, protection can be achieved by painting the trunks white. Trunk guards should be white or light in color, and should extend from a couple inches below the soil surface to 2 or 3 feet above the ground. Trunk guards that are left in place must permit the trunk to grow without restraint, or they will girdle and kill the tree.
Mice and other small rodents will, during a hard winter, nibble bark around the base of a tree. This can result in girdling, and subsequent death of the tree. Trunk guards placed to protect against freeze injury will prevent rodent damage, but when these guards are removed some form of bark protection should be put in place. Some growers use short sections of trunk guard material, buried in the ground and protecting the lower 6 to 8 inches of the trunk.
Deer can cause devastating damage to fruit trees during the winter. They gnaw at the bark, tearing off ragged strips, and breaking tree limbs as they feed. Some growers claim success with various kinds of deer deterrents, but these remedies are generally ineffective over the long term. Fruit trees must be protected from deer with sturdy wire fencing, 8 feet high on 10-foot posts, or a professionally installed electric fencing system.
Maintenance pruning should be done as late in winter as possible. Pruning done in early winter risks winter injury. Remember that heavy winter pruning, done while the tree is dormant, results in a burst of energetic leafy growth in spring. The stored energy used by the tree will decrease fruit production in the following season.
Prune lightly to encourage a sound structure, and make sure the cuts are clean — use sharp tools, and sterilize the blades between cuts with alcohol. Do not paint anything on the cut surfaces -- the wound will heal better if it is left uncovered.
Young trees being trained to a central leader system should be headed back at about 30 inches above the ground.
Winter winds can harm young trees. Apple trees must be supported for the first few years of their growth. Twin stakes and non-abrasive straps, placed each side of the trunk when the tree is planted, are the best way to provide support while allowing a healthy amount of movement.